Phase-sensitive detection in Raman tweezers

The authors discuss on a method to acquire the Raman spectrum of a single optically trapped particle. The method makes use of two laser beams: the first laser traps the particle and moves it back and forth in a plane perpendicular to the laser beam propagation; a second laser acts as Raman probe and it is fixed in space. The Raman spectrum is obtained by phase detecting the backscattered Raman photons using a lock-in amplifier. Within this approach, the background due to the scattering of the environment is completely removed. The authors apply this method to 4.25 ?m diameter polystyrene beads in aqueous solution.